Characterization of the Frequency and Muscle Responses of the Lumbar and Thoracic Spines of Seated Volunteers During Sinusoidal Whole Body Vibration

Abstract

Whole body vibration has been postulated to contribute to the onset of back pain. However, little is known about the relationship between vibration exposure, the biomechanical response, and the physiological responses of the seated human. The aim of this study was to measure the frequency and corresponding muscle responses of seated male volunteers during whole body vibration exposures along the vertical and anteroposterior directions to define the transmissibility and associated muscle activation responses for relevant whole body vibration exposures. Seated human male volunteers underwent separate whole body vibration exposures in the vertical (Z-direction) and anteroposterior (X-direction) directions using sinusoidal sweeps ranging from 2 to 18 Hz, with a constant amplitude of 0.4 g. For each vibration exposure, the accelerations and displacements of the seat and lumbar and thoracic spines were recorded. In addition, muscle activity in the lumbar and thoracic spines was recorded using electromyography (EMG) and surface electrodes in the lumbar and thoracic region. Transmissibility was determined, and peak transmissibility, displacement, and muscle activity were compared in each of the lumbar and thoracic regions. The peak transmissibility for vertical vibrations occurred at 4 Hz for both the lumbar (1.55 ± 0.34) and thoracic (1.49 ± 0.21) regions. For X-directed seat vibrations, the transmissibility ratio in both spinal regions was highest at 2 Hz but never exceeded a value of 1. The peak muscle response in both spinal regions occurred at frequencies corresponding to the peak transmissibility, regardless of the direction of imposed seat vibration: 4 Hz for the Z-direction and 2-3 Hz for the X-direction. In both vibration directions, spinal displacements occurred primarily in the direction of seat vibration, with little off-axis motion. The occurrence of peak muscle responses at frequencies of peak transmissibility suggests that such frequencies may induce greater muscle activity, leading to muscle fatigue, which could be a contributing mechanism of back pain.

Whole-body Vibration at Thoracic Resonance Induces Sustained Pain and Widespread Cervical Neuroinflammation in the Rat

“…1, 2, S1). This resonance is consistent in terms of order of magnitude with spinal responses of other species: human (approximately 4 Hz) [10,64], primate (5-14 Hz) [72], and rabbit (4.5 Hz) [80]. Spinal muscle activity in the human is also maximal at resonance [10].…”

“…Findings are important given that spinal pain is one of the most common causes for evacuation from military operation with 86% of those not returning to deployment [20,57]. Although our study provides important physiologic context for exposure to vibration at resonance and there is anecdotal evidence supporting the assertion that service members are regularly exposed to spinally resonant frequencies [10], there still remains a need for field data relating exposures to pain.…”

“…4-7). Although this study limited assessments to DRG and the spinal cord, other nonneural tissues are susceptible to WBV injury [10,40]; regardless, the largest modifications are expected after WBV at resonance. The peak transmissibility of the thoracic spine of the rat for spinally directed WBV is between 8 and 10 Hz with greater cervical deformations at 8 Hz than 15 Hz when acceleration is constant and displacement is varied (Figs.…”

The Use of Body Worn Sensors for Detecting the Vibrations Acting on the Lower Back in Alpine Ski Racing

“…In accordance with the international standard ISO 2631 (ISO, 1997), the inertial acceleration data was filtered in the frequency domain prior to computing the RMS according to the ISO filter specifications [frequency weighting Wk (vertical direction) with k = 1]. This filter amplifies accelerations at frequencies close to the resonant frequency of the spine [~4–10 Hz according to Izambert et al (2003), Guo et al (2009), Guo et al (2011), and Baig et al (2014); (Figure 2)]. RMS was then equal to the RMS value of this filtered acceleration.…”

“…Third, there is strong scientific evidence that excessive exposure to whole-body vibrations, particularly at frequencies close to the resonant frequency of the spine [~4–10 Hz according to Izambert et al (2003), Guo et al (2009), Guo et al (2011), and Baig et al (2014)], increases the risk of structural deteriorations/abnormalities of the spine and of developing low back pain (Hill et al, 2009; Burström et al, 2015). For that and other reasons, there are international standards such as, ISO 2631 (ISO, 1997) or Directive 2002/44/EC of the European Union (EU, 2002) that define minimum health and safety requirements for the exposure of workers arising from whole-body vibrations (Griffin, 2004).…”